Planar and Outerplanar Spectral Extremal Problems based on Paths

Abstract: Let SPEX$\mathcal{P}(n,F)$ and SPEX$\mathcal{OP}(n,F)$ denote the sets of graphs with the maximum spectral radius over all $n$-vertex $F$-free planar and outerplanar graphs, respectively. Define $tP_l$ as a linear forest of $t$ vertex-disjoint $l$-paths and $P_{t\cdot l}$ as a starlike tree with $t$ branches of length $l-1$. Building on the structural framework by Tait and Tobin [J. Combin. Theory Ser. B, 2017] and the works of Fang, Lin and Shi [J. Graph Theory, 2024] on the planar spectral extremal graphs without vertex-disjoint cycles, this paper determines the extremal graphs in $\text{SPEX}\mathcal{P}(n,tP_l)$ and $\text{SPEX}\mathcal{OP}(n,tP_l)$ for sufficiently large $n$. When $t=1$, since $tP_l$ is a path of a specific length, our results adapt Nikiforov's findings [Linear Algebra Appl. 2010] under the (outer)planarity condition. When $l=2$, note that $tP_l$ consists of $t$ independent $K_2$, then as a corollary, we generalize the results of Wang, Huang and Lin [arXiv: 2402.16419] and Yin and Li [arXiv:2409.18598v2]. Moreover, motivated by results of Zhai and Liu [Adv. in Appl. Math, 2024], we consider the extremal problems for edge-disjoint paths and determine the extremal graphs in $\text{SPEX}\mathcal{P}(n,P{t\cdot l})$ and $\text{SPEX}\mathcal{OP}(n,P{t\cdot l})$ for sufficiently large $n$.